The subject of the present invention is crosslinking resin mixtures which are liquid at temperatures under 100.degree. C. and are prepared on the basis of polymers containing hydroxyl groups and having a molecular weight of 500 to 20,000.
It is known that alkoxysilanes react readily with moisture. This tendency toward hydrolysis is useful in many ways, the adhesion-mediating action of the organosilanes between different kinds of materials being worthy of special note (see "Handbook of Adhesives", 2nd Edition, by Irving Skeist, pp. 640 sqq., Van Nostrand-Reinhold Co., 1977).
The hydrolysis of the alkoxysilanes with the formation of siloxane bonds has also been widely used for the crosslinking of polymers. In DE-OS No. 2,314,757 there is described a binding agent of which at least 50% by weight consists of a copolymer having lateral alkoxysilane groups. U.S. Pat. No. 4,291,136 describes a water-hardening, silane-modified alkylene-alkyl acrylate copolymer. Such copolymers, however, must be fabricated either from solution in organic solvents or with machines commonly used for plastic fabrication, such as extruding machines.
Attempts have already been made to produce liquid polymers containing no solvents, which can be crosslinked by the addition of water or by reaction with atmospheric moisture. In U.S. Pat. No. 3,632,557, the starting products are liquid polymers having terminal hydroxyl groups, which are transformed by reaction with excess diisocyanate to a polymer having terminal isocyanate groups. These are then reacted with an aminosilane, preferably gamma-aminopropyltrimethoxysilane, so that finally a liquid polymer results which is provided stoichiometrically, i.e., quantitatively, with alkoxysilyl terminal groups. Such liquid polymers set with atmospheric moisture to form rubber-like products which, however, suffer from a number of undesirable properties, as described very extensively in U.S. Pat. No. 3,971,751. Their poor elongation at rupture, and above all their extremely poor resistance to continued tearing, are serious disadvantages, and is found to a more or less great extent in all such materials which are terminated stoichiometrically with alkoxysilyl groups. This is equally true of the products described in U.S. Pat. Nos. 3,979,344 and 4,222,925. This disadvantage is largely overcome by the procedure described in U.S. Pat. No. 2,971,751; the process therein described, however, is so complex that the results described can be obtained better, and especially much more simply, by other methods.
The reactivity of alkoxysilanes with the hydroxyl groups of polymers containing hydroxyl rather than with water is utilized in the method that is the basis of Japanese Pat. No. 56084751: liquid polymers with terminal hydroxyl groups are hardened with alkoxysilanes at elevated temperature (150.degree. C.) and elevated pressure (150 bar) to form elastomeric moldings. This process can be performed only with the application of pressure because, at the temperatures at which the transesterification is performed, the alkoxysilanes otherwise would volatilize and thus would be unable to act. For applications in which the material is in a thin layer, as for example in any kind of coatings, varnishes, glues, etc., this technology is entirely inapplicable.